Doorstep Astronomy: See the Summer Triangle

The Summer Triangle dominates the night sky in the summer months. The triangle is made up of three bright stars from three separate constellations. Vega in Lyra, Altair in Aquila and Deneb in Cygnus. This map shows the Eastern sky as seen on July 6 at 10 p.m. local time from mid-northern latitudes.Credit:

Well up in
the eastern sky during the late-evening hours is the "Summer Triangle,"
not a constellation, but a very noteworthy star pattern of the summer sky
consisting of three bright stars from three constellations.

Brightest
of the three is Vega,
in the constellation of Lyra, the Lyre. Next in brightness is Altair in Aquila, the Eagle. Finally, there is Deneb in Cygnus, the Swan.

To our
eyes, Altair appears to shine only half as bright as Vega and Deneb seems only
one-third as bright as Vega. So at first glance, Vega appears to rule this
section of the sky. In reality, however, a bit of celestial deception is at
work here, for Vega is appears very bright chiefly because of its proximity to
us. Vega is 25 light-years away, so when you are looking at it tonight, you are
really seeing it as it was back in 1982.

Altair is
17 light-years away, closer than Vega, yet appearing somewhat dimmer. That tells
us that in terms of overall luminosity between the two, Vega is indeed the
brighter star: it is 48 times brighter than our sun, while Altair is only 12
times brighter.

A true
giant

But what
about the third star in the triangle, Deneb? Here, there is no comparison
between either Altair or even Vega, for Deneb is one of the greatest supergiant
stars known. It shines about 80,000 times brighter than our sun, but because of
its vast distance from us  1,467 light-years away  it appears as just a
fairly conspicuous but by no means noteworthy star.

If it were
somehow possible to move Deneb to Vega's distance from us, it would then appear
to shine 16 times brighter than Venus is now, and capable of casting distinct
shadows and be visible even in the daytime. Conversely, if we were to move Vega
out to where Deneb is, it would shine only as bright as a ninth-magnitude star;
you would need a star atlas to precisely locate it, and it would only be
visible in good binoculars or a telescope.

With the bright moon now pretty much out of the evening sky
this week, there is no better time than now to observe the beautiful summer
Milky Way. With a good pair of binoculars or a telescope you can now observe
millions of sparkling little stars that make up this glowing, irregular belt of
luminosity. It appears to arch from the north-northeast to the south-southeast,
with its brightest and most spectacular region running across the Summer
Triangle and beyond toward the south-southeast horizon.

There appears to be a great black rift (sometimes called the
"bifurcation") dividing it into two streams, beginning with Cygnus
and extending down toward the south. Also in Cygnus is the black void known as
the Northern Coal Sack. This Coal Sack and the Rift are not holes in the Milky
Way, but rather are vast clouds of dust drifting in interstellar space which,
from our Earthly perspective, presents a solid and impenetrable curtain between
us and the more distant stars.

Stuff of legends

There have been many stories, myths and legends told about
the Milky Way in many different cultures.

This particular weekend, the Milky Way involves a poignant
Japanese legend. The star Vega represented Orihime, who
produced brilliantly colored fabrics. Across the "Heavenly River" (the Milky Way), Altair represented the cowherd Kengyu. After meeting each other
they received divine permission to marry, whereupon both abandoned their
occupations. This angered the gods who separated them and sent them back to
their original jobs on opposite sides of the heavenly river. The couple
however, received permission from the gods to get together for one night each
year.

That special night is July 7  but only if the sky is clear!

As a result, the evening of July 7 has evolved into a
young-people's holiday in Japan called Tanabata. Prayers are then offered for
clear skies so that Orihime and Kengyu, the star-crossed lovers can be
reunited. One popular Tanabata custom is to write one's wishes on a piece of
paper, and hang that piece of paper on a specially erected bamboo tree, in the
hope that the wishes come true.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other
publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.